Tube rolling mill employing a tapered mandrel and a cluster of rolls that each have specially designed tube contacting grooves

ABSTRACT

A tube rolling mill having two sets of three rolls each which are reciprocatingly driven along a length of a tube supported by a tapered mandrel. Each roll is forced against the tube by its individual cams each having a surface with one or more tapers which are related to multiple mandrel taper in a manner to provide reduction in both wall thickness and inside diameter of the tube. Each roll is provided with a tube contacting groove having in cross-section a central arc with a radius of curvature substantially equal to the smallest radius of that portion of the tube contacted by the roll, with either side of the central arc joined by lines tangent thereto with large radii of curvature chosen so that each roll contacts a tube in two zones around its circumference. The rolls are pressed against the tube upon the urging of the tapered cam surface against a roll trunnion. The radius of the trunnion is carefully chosen to control longitudinal forces transferred to the tube by the roll.

ilnited States Patent Russel [54] TUBE ROLLING MILL EMPLOYING A TAPERED MANDREL AND A CLUSTER OF ROLLS THAT EACH HAVE SPECIALLY DESIGNED TUBE CONTACTING GROOVES [72] Inventor: Richard E. Russel, Paoli, Pa.

[73] Assignee: Superior Tube Company, Norristown, Pa.

[22] Filed: July 7, 1970 [21] Appl. No.: 52,838

Related US. Application Data [63] Continuation-impart of Ser. No. 845,832, July 29, 1969, abandoned.

[52] US. Cl ..72/208, 72/214 [51] Int. Cl. ..B21b 17/06 [58] Field of Search ..72/208, 209, 234, 370, 214

[56] References Cited UNITED STATES PATENTS 3,517,537 6/1970 Cofer ..72/234 3,566,655 3/1971 Scholten et al. ..72/209 2,161,065 6/ 1939 Krause ..72/209 2,101,959 2/1937 Schulz ..72/224 [451 Sept. 5, 1972 1,193,001 8/1916 Edwards ..72/235 Primary Examiner-Milton S. Mehr AttorneyWoodcock, Washbum, Kurtz & Mackiewicz [57] ABSTRACT A tube rolling mill having two sets of three rolls each which are reciprocatingly driven along a length of a tube supported by a tapered mandrel. Each roll is forced against the tube by its individual cams each having a surface with one or more tapers which are related to multiple mandrel taper in a manner to provide reduction in both wall thickness and inside diameter of the tube. Each roll is provided with a tube contacting groove having in cross-section a central arc with a radius of curvature substantially equal to the smallest radius of that portion of the tube contacted by the roll, with either side of the central are joined by lines tangent thereto with large radii of curvature chosen so that each roll contacts a tube in two zones around its circumference. The rolls are pressed against the tube upon the urging of the tapered cam surface against a roll trunnion. The radius of the trunnion is carefully chosen to control longitudinal forces transferred to the tube by the roll.

15 Claims, 6 Drawing figures PATENTEDSEP 5 I972 3 68 8 540 sum 2 0F 4 Fig. 2

PKTENTED SEP 5 I97? 3 688 5O SHEET k 0F 4 DIRECTION OF ROLL TRAVEL CROSS-REFERENCE TO RELATED APPLICATIONS This is a continuation-in-part of application Ser. No. 845,832, filed July 29, 1969, now abandoned, and is related to application Ser. No. 845,833, filed July 29,

1969, now U.S.Pat. No. 3,611,775.

BACKGROUND OF THE INVENTION This invention relates generally to a method and apparatus for reducing and elongating metal tubing and more particularly to improvements in a method and apparatus for reducing and elongating metal tubing with the use of rolls.

Metal tubing is used in a wide variety of environments and for many different applications. This requires that the tubing be available with a wide variety of inside and outside diameters and wall thicknesses. In order to effectively utilize the economies of mass production, metal tubing is initially manufactured in only a few standard sizes. This makes it necessary to modify tubing of a standard manufactured size to obtain a size needed for a particular application involving smaller quantities of tubing than can be economically manufactured directly.

A machine for reducing tubing of a standard manufactured size is described by Krause in the Iron and Steel Engineer, Aug. 1938, pages 16-29, and in several patent publications such as U.S. Pat. Nos. 2,161,064, 2,161,065, and 2,223,039. The machines described therein utilize a single set of two rolls operating against a mandrel supported tube to effect the tubes reduction. Each roll is rolled along a working length of the tube in response to the movement of a cam for controlling the pressures exerted by the roll against the tube. These machines provide for only a limited reduction in wall thickness without significant reduction of the inside diameter of the tube.

In addition, there have been several disclosures by Argonne National Laboratories resulting to similar machines. In an Argonne paper ANL-MS-990, dated Mar., 1968, entitled Fabrication of Vanadium Alloy Tubing by Mayfield and Karasak, a rolling mill is disclosed which is capable of limited inside diameter tube reduction. In an Argonne print MY-B-2700-A'19D, part 33D, a roll groove design is described wherein the roll groove has a center arcuate surface terminating in two tangent straight surfaces at its edge. The radius of the arcuate surface matches the largest radius of a tube length to be worked by the roll, whereby the tangent fiat surfaces serve only as side relief for non-uniform deformation of the tube. Such single point contact of the roll groove with a tube is not an efficient working arrangement.

Several publications by Russian authors have described similar tube rolling mills. These disclosures, however, tend to show that three or six roll tube rolling machines now in existence have merit in reducing wall thickness while possessing only limited capability for reducing the inside diameter. Such machines do not enjoy a wide application to the tube industry since the majority of such applications require substantial reduction in diameter.

Therefore, it is a primary object of this invention to provide a tube rolling mill capable of making large reductions in both wall thickness and inside diameter.

It is also an object of this invention to provide a tube rolling mill with a high efficiency and feed rate.

It is a further object of this invention to provide a tube rolling mill with a high degree of reliability and additionally with a capability of producing reduced tubes with a wide variety of wall thicknesses and inside diameters.

It is yet another object of this invention to provide a tube rolling process that produces a reduced tube of improved quality.

SUMMARY OF THE INVENTION These and additional objects are accomplished by a machine according to this invention in which a number of elements are combined to reduce tube wall thickness and additionally reduce the tube inside diameter without a machine complexity any greater than exists in other tube reducing machines. Two sets of three rolls each are provided for contacting a tube with grooves provided around the circumferential edges of each roll. Each roll is rotatably attached to a roll housing and is reciprocated thereby along a working length of a mandrel which is designed to fit within a tube to be reduced. Each set of three rolls is clustered around the tube with their axes of rotation located in a plane substantially perpendicular to the length of the mandrel and displaced from each other. Two sets of rolls are spatially fixed relative to one another in a direction along the length of the mandrel by the roll housing. One set of rolls reduces the tube an intermediate amount and the second set of rolls completes the reduction to the dimensions desired. The axes of rotation of the rolls of one set are displaced 60 from the axes of rotation of the rolls of the other set. The mandrel is tapered along at least a portion of its working length over which the roll housing reciprocates the tube contacting rolls. The rolls are urged toward the mandrel by an individual cam surface for each roll, said cam making a contact with a roll along its surface substantially opposite the tube contacting surface. The cam and mandrel are shaped so that each roll contacts a tube to be reduced during at least a portion of each reciprocating stroke of the roll housing.

The tube contacting groove around the outer edge of each roll has a uniform shape in cross-section at any point around the circumference of the roll. As each roll is pressed against a tube and rolled therealong, the outside diameter of the tube becomes smaller. The crosssectional shape of the groove includes an arcuate portion in the middle thereof having a radius of curvature substantially equal to or slightly less than the smallest radius of the outside surface of a tube portion contacted by that particular roll. On either side of this arcuate portion of the groove is in cross-section a substantially flat portion extending on either side to the edge of the groove, the flat portions joining the arcuate portion as a tangent thereto. The advantage of this arrangement is that at the beginning of the working stroke, a tube contacts the roll groove at two portions (zones) therealong and does so throughout the working stroke. This two-zone contact of each roll groove makes more efficient use of the working stroke. With a six roll machine, a tube is contacted at twelve zones therearound, thereby providing considerably more surface area contact between the rolls and the tube than in other machines. This results in a greater effective volume of metal reduction per rolling stroke and a better quality reduced tube.

The cams and the mandrel are cooperatively shaped in a manner to provide a maximum bite of each roll at its contact zones into the tube to be reduced. A maximum bite is provided throughout wall reduction portions of the working stroke and is slightly less than the bite which will strain any portion of the tube to its point of rupture. At the beginning of the working stroke, this bite is much larger than at the end of the working stroke. This maximum bite at points along the tube is dependent upon, among other things, the material of the tube being reduced. Such a maximization of the roll bite additionally improves the reduction possible in a given working length of the machine.

The cams are attached to a cam housing which is reciprocated along the length of the mandrel in order to contact the rolls and guide them against the tube without significant slippage between each roll and its cams. Instead of contacting the outer circumference of the roll, the cams are designed to contact trunnions provided on either side of each roll. This has the advantage that a larger bearing surface may be provided to prolong the life of the rolls and the cams, as well as to allow an adjustment which is beneficial to the tube rolling process. The roll housing and cam housing are driven from a common motor source with a constant ratio of velocities therebetween. This velocity ratio and the relationship between the radius of the rolls trunnions and the radius of that portion of the roll groove contacting a tube at a specific point thereof determines the degree to which the roll will tend to apply torque to the tube and cams, and thereby determines the tendency of the groove to slip against the surface of the tube when the tube is adequately held against lateral movement.

For a given configuration, the torque forces causing such slippage vary along the working length of the tube since the effective radius of the roll varies therealong. It has been found desirable to control the torque forces tending to slip the roll against the tube in a manner to compensate for the horizontal forces applied to the tube by a roll biting into the tube wall. Such compensation allows mandrel and tube gripping devices to more accurately position the tube. The compensation also allows larger roll bites without creating forces of a magnitude that cause machine elements to fail, thereby accomplishing more tube reduction in a given working length and amount of time.

When a rolling mill employing two sets (stages) of rolls that simultaneously contact a tube is used, it has been found desirable to balance the torque forces between rolls of each set to compensate for tube elongation.

For a more detailed understanding of the invention and for further objects and advantages thereof, reference is made to the following description of its preferred embodiments taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates in a partially exploded view a rolling mill according to the present invention;

FIG. 2 is a cross-sectional view of FIG. 1 taken through the first stage (set) of rolls at 2-2;

FIG. 3 is a cross-sectional view of FIG. 1 taken through the second stage (set) of rolls at 3-3;

FIG. 4 schematically illustrates the operation of the primary operating components of the rolling mill illustrated in FIGS. 1, 2 and 3;

FIG. 5 shows the shape of a preferred tube contacting groove of a roll for use in the rolling mill shown in FIGS. I3; and

FIG. 6 shows a side view of a preferred roll illustrating an optimum trunnion radius for use in the rolling mill shown in FIGS. 1-3.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. I, a cam housing 11 is reciprocated relative to a machine frame 13 along a slide 15 in substantially a straight line. An electric motor 17, also attached to the frame 13, drives a flywheel 19. A rod 21 (partially broken away) is connected between the flywheel 19 at a crank pin 20 and the cam housing 1 l to convert rotary motion of the flywheel to reciprocable motion of the cam housing. Within the cam housing 11 is a reciprocatable roll housing 23, shown removed from the cam housing for clarity of illustration. A pinion gear engages a rack 27 that is rigidly attached to the frame 13. A second pinion gear 26 engages a rack 29 that is rigidly attached to the cam housing 11. The pinion gears 25 and 26 are concentric about a common axis of rotation 30 and are nonrotatable relative to each other. The reciprocable motion of the axis of rotation 30 of the pinion gear 25 is communicated to the roll housing 23 by means of a connecting rod 31 (shown herein as two sections since the roll housing 23 is shown removed from the cam housing 11). The cam housing 11 has a maximum reciprocation stroke distance that is equal to the diameter of the circular path taken by the crank pin 20. From the geometry of the driving arrangement of FIG. 1, the roll housing 23 has a maximum reciprocation stroke distance that is equal to the maximum stroke of the cam housing 11 multiplied by the diameter of the pinion gear 25 and then divided by the sum of the diameters of the pinion gears 25 and 26. The use of two pinion gears having different radii as herein has the effect of increasing the length of the working zone along the tube without increasing the stroke length of the cam housing. It should be noted that although the double pinion gear arrangement herein is very convenient for controlling the maximum relative cam housing and roll housing stroke distances, and thereby their relative velocities, other specific mechanical arrangements, such as one employing levers, may also be employed for the same purpose.

Another aspect of the geometry of the arrangement in FIG. 1 is that the cam housing stroke distance is equal to the sum of the roll housing stroke and the working stroke length of the cams (the distance along each cam that contacts a roll trunnion) within the cam housing. It follows, then, that the cam length contacted by each roll bears the same relationship to the roll housing stroke as a ratio of the diameter of the cam housing pinion gear 26 to the diameter of the roll housing pinion gear 25.

A tube 33 to be reduced is inserted through an opening 35 of the roll housing 23, and is carried by a mandrel 37. The mandrel 37 is rigidly held fixed relative to the machine frame 13 by an appropriate gripping device 36, which also provides for removing the mandrel. An appropriate apparatus 38 is provided for positively gripping the tube 33 linearly over a working length of the mandrel 37, and further to incrementally rotate the tube. The apparatus 38 causes such feeding and rotation of the tube being reduced at specificportions of the reciprocating cycle, as is hereinafter discussed.

FIGS. 2 and 3 better show the relationship of tube deforming rolls and the cam housing as sectional views of FIG. 1. A first set of rolls 39, 41 and 43, shown in FIG. 2, are held in the roll housing 23 with their axes of rotation lying substantially in a plane perpendicular to the mandrel 37 and making an angle of 120 with each other. Similarly, a second set of rolls 45, 47 and 49, shown in FIG. 3, are held by the roll housing 23 in the position that their axes of rotation lie substantially in a plane perpendicular to the mandrel 37 and at a spatially fixed distance along the length of the mandrel from the plane in which the axes of rotation of the first set of rolls 39, 41 and 43 lie. Furthermore, the axes of rotation of the two sets of rolls are angularly displaced from each other by 60.

Although held by the roll housing 23 against movement relative thereto in the direction of its reciprocation, each roll is free to move in a direction normal to the mandrel. Each roll is resiliently urged by a set of springs (such as springs 49 and 50, each held within a roll guide) out of the roll housing 23 and against its associated cam surfaces. Alternatively, the rolls may be hydraulically urged against their associated cams. Each roll has a trunnion formed on either side thereof, such as trunnions 51 and 53 on either side of the roll 45. Each roll is associated with a pair of cam tracks upon which its pair of trunnions ride. The two cams associated with each roll are designated herein with the same number as the roll but with an asterisk placed after the reference number of one of the cam tracks and a double asterisk placed after the number referring to the other of the cam tracks. The cams are long metal bars shaped in a manner discussed hereinafter and rigidly attached to the cam housing 11. This attachment is accomplished through a recessed member for each pair of cams, such as a recessed member 55 which is shaped to support the cams 45* and 45**. Notice that the cams 45* and 45** each have a sloped side surface which allows fastening them to the recessed member 55 by a wedge 57 which is attached to the recessed member by a threaded fastener 58.

It should be noted with reference to FIGS. 1, 2 and 3, the ease with which the cam surfaces may be replaced in the cam housing 11 and also the ease with which the rolls may be replaced in the roll housing 23. A given pair of cams are removed by removing their associated wedge. The rolls are merely lifted out of the roll housing 23 when the roll housing is removed to a position as illustrated which is out of the cam housing 11. The mandrel 37 is also easily removed. These features allow quick conversion of the tube rolling mill to receive raw tubes of various sizes and also to produce finished tubes with various wall thicknesses and inside diameters.

The schematic diagram of FIG. 4 illustrates operation of the rolling mill illustrated in FIGS. 1, 2 and 3.

The mandrel 37 is shown in cross-section along its length which includes a tube working zone BJ wherein at all points therealong the tube 33 is contacted by one or both sets of rolls to accomplish reduction either in wall thickness or inside diameter or both. The mandrel has a diameter d at its large end which is something slightly less than the inside diameter of the starting tube 33, thereby allowing the tube to be slid easily over the mandrel. The small end of the mandrel has a diameter d which is substantially equal to the desired inside diameter of the reduced tube. The mandrel 37 is gradually tapered within the tube working zone from one of these diameters to the other. This taper is significantly in excess of that required for tube relief. The diameters d and d may differ by 20 or 30 percent or more, depending on the tube inside diameter reduction desired.

In order to demonstrate the cooperation between the cams and the mandrel, one roll from each of the two sets of rolls is shown in FIG. 4 as if they operated in the same plane so that the relationship between them and their cooperation in reducing the tube are illustrated. Rolls 39 and 45 are illustrated in FIG. 4 along with their associated cams 39* and 45*, respectively. The axis of the roll 39 reciprocates along the tube between positions A and I with a distance therebetween equal to the stroke distance of the roll housing 23 (not shown in FIG. 4) in which the roll 39 is journaled. Similarly, the axis of the roll 45 reciprocates along the tube between positions D and K. The cams 39* and 45* are attached to the cam housing 11 (not shown in FIG. 4) and thereby are reciprocatably driven at a greater velocity than the axis of the rolls, as described hereinabove. The cam 39* contacts a trunnion 40 attached to the roll 39 and the cam 45* contacts the trunnion 51 of the roll 45. The shape of the cams and of the mandrel determine the displacements of rolls downward against the tube to bring about a desired deformation of the tube.

Consider a single working stroke wherein the rolls and cams of FIG. 4 move from their far left hand position to the far right and back again. This represents the extent of movement brought about by a single revolution of the flywheel 19 of FIG. 1. The roll 39 begins at the position A and the roll 45 begins at the position D. As shown by the dashed lines, the roll 39 contacts the tube 33 for the first time at about the position B and the roll 45 contacts the tube 33 for the first time at approximately the position F. Proceeding further to the right, the cooperative shapes of the cams and the mandrel allow the roll 39 to be lifted from the tube 33 at about the position F, as shown by the path [39] of the roll, away from the tube. Similarly, the roll 45 is caused to be lifted from the tube 33 at about the position J, as shown by the path [45] of the roll, away from the tube. The roll 39 arrives at the position I at the same time the roll 45 arrives at the position K to complete the first one-half of the working stroke. The rolls 39 and 45 then move back to their beginning positions A and D, respectively, to complete one working stroke cycle. It may be noted that the cam working lengt as used herein is a horizontal projection of the length of a cam surface contacted by the trunnion. With reference to the cam 39*, the cam working length thereof is the horizontal distance between points A, and I The tube 33 is advanced (fed) by the apparatus 38 an increment to the right while the rolls are drawn away from contact with the tube, either at one or both ends of the working stroke. Similarly, the apparatus 38 rotates the tube 33 either at one or both ends of the working stroke. It is preferred to rotate the tube through a small angle at each end of the working stroke because a smoother finished tube is the result. The shape of the tube 33 shown in FIG. 4 within the working zone represents the finished shape thereof after a working stroke and before the tube is fed an increment in preparation for the next working stroke.

There are many specific cam and mandrel shapes that may be utilized depending upon the specific tube reduction desired. FIG. 4 illustrates a preferred arrangement for major inside diameter reduction. The following tabulation describes the work done by the roll 39 within the working zone between lettered positions along the length of the tube:

Between B-C: The tube is reduced to intimate contact with the mandrel.

Between C-E: Primarily tube diameter reduction is accomplished by the roll 39.

Between E-F: Primarily wall reduction is accomplished by the roll 39.

The following tabulation describes the work concurrently performed by the roll 45 within the working zone between lettered positions along the length of the tube:

Between FG: Primarily wall reduction performed by the roll 45.

Between G-I-I: Primarily wall reduction performed by the roll 45 but with a lesser bite into the tube than between F-G.

Between H-J: This is a finishing zone where there is substantially no taper to the mandrel 37 and with very little bite of the roll into the tube.

To accomplish the above-noted specific tube reductions at various points within the tube working zone, the mandrel has one or more straight line tapers. The cams are shaped cooperatively therewith, each having a plurality of straight line tapers. The cams of FIG. 4 have their roll contacting surfaces marked with subscripted letters corresponding to the lettered positions along the tube. For example, when the roll 39 is positioned at E along the tube, the cam 39* is contacting the trunnion at position E Straight line tapers are preferred for the cams and the mandrel since they are easy to machine, although continuous curves may also be employed.

The description herein with respect to FIG. 4 is exemplary only with various changes in the specifics thereof being possible. For example, if major inside tube diameter reduction is not required, the portion B F, of the cam 39* may be shaped differently relative to the portion B-F of the mandrel than as shown to effect tube wall reduction between B-F instead of tube diameter reduction. Also, the elements may be designed so that the rolls 39 and 45 overlap in their work zones along a portion of the tube, preferably with dissimilar cam tapers acting on the two rolls in this common length of the tube. Also, certain applications may require only a single taper along a working length of each of one set of cams. Furthermore, in those cases where little inside diameter reduction is desired, the cams and rolls described herein may be used with a mandrel having little or no taper.

Along any of the portions of tube length wherein substantial wall thickness reduction is desired, the controlling cam and mandrel tapers are designed for a bite of the rolls into the tube at each point within this portion that is approximately the same percentage of the wall thickness at that point before the roll. This maximizes the efficiency of the wall thickness reduction, thereby allowing more reduction to be accomplished in a shorter portion of the working stroke. Multiple straight line tapers on the cams may be employed to ap proximate this constant percentage although continuous curved cam surfaces are more exact. The amount of tube feed for each stroke is then adjusted to a maximum for a given tube material just short of that which ruptures the tube, thereby maximizing productivity of the machine.

A preferred tube contacting groove is illustrated in FIG. 5 for the rolls of a rolling mill illustrated with respect to FIGS. 1-3. The groove shape is uniform in cross-section at any radial plane thereof. The groove cross-section is shown on a roll 79 which represents relative roll groove dimensions for any roll shown in FIGS. 1-3 for the purpose of describing roll groove design. In the center of the groove is an arcuate portion 81 having a center of curvature at a point 83. Joining either side of the arcuate center portion 81 as tangents thereto at its end points 85 and 87 are straight line segments 89 and 91 which extend to the groove outside edges 97 and 99, respectively. The arcuate portion 81 extends for an angular distance d) on either side of a center line.

The radius of curvature of the arcuate portion 81 is made substantially equal to or slightly less than the smallest outside tube radius the roll is designed to contact, such outside tube radius being represented by a solid circle 93. This represents the radius of the finished tube for the roll 45 shown in FIG. 4 and the radius of the tube at location F for the roll 39. A. circle (FIG. 5) represents the largest outside tube radius which the roll groove is designed to contact, that of the beginning tube for the roll 39 of FIG. 4 and that of the tube at position F for the roll 45.

This roll groove design provides two zones of contact for each roll against the outside of the tube between the tubes larger portion (95) and substantially until its smallest portion (93). Such two-zone rolling accomplishes more reduction in a given working zone of a tube when compared to a roll groove providing only one zone of contact with the tube. Non-uniform tube wall strain is reduced as well as resulting degradation of tube quality. Also, required rolling forces, and thus machine wear, are reduced. To optimize these advantages, the radius of the arcuate center portion 81 of the roll groove may be made I or 2 percent less than the smallest outside tube radius to be contacted by the roll groove, thereby extending two zone rolling over the entire length of the tube contacted by the roll whereby roll life is extended. FIG. 5 illustrates such a preferred roll that is designed to contact the tube at two zones throughout the stroke. A rolling radius r of the roll '79 along the tube varies between R (contacting tube portion 95) and R (contacting tube portion 93) during each tube reducing stroke. An arcuate center portion 81 with a radius significantly smaller than the finished tube outside radius (in the extreme the groove becomes V-shaped) results in a finished reduced tube surface that is irregular and rough.

When the roll 79 of FIG. is pressed against a tube during the rolling thereof, it bites into the tube at its zones of contact with the roll groove. This results in the extreme edges 97 and 99 becoming closer to the tube. It is desirable to maintain a clearance between the outside edges 97 and 99 and the tube outside wall to prevent scoring or grooving of the tube. Therefore, a roll having a given groove is limited to a maximum tube bite of something less than the distance between the groove edges 97 and 99 and the largest tube portion to be contacted by the roll, a distance g shown in FIG. 5. This clearance of the edges 97 and 99 is increased by decreasing the angle 1). However, as qb is reduced to small values, the two zones of tube contact become closer together thereby increasing wall strain and the possibility of localized tube failure. Therefore, there is a trade-off between the desire to maximize the bite of the roll into the tube and the desire to maintain the advantage of two zone rolling. For a given tube material, there is an optimum angle which allows the roll to take the most efficient maximum bite into the tube, thereby resulting in the most rapid feed rate of the tube through the machine. An angle of from 30 to 38 is satisfactory for most common tube materials and specific types of reduction.

The tangential portions 89 and 91 of the roll groove are shown in FIG. 5 as straight lines. However, these portions of the groove may, alternatively, be given a curvature with one or more finite radii of curvature. The edges 97 and 99 of the roll groove may be curved to eliminate the sharp comer which can damage the tube surface. The remaining segments of the tangential portions 89 and 91 may then remain straight or may be curved slightly, either concave or convex. If part of the tangential portions is made concave, tube wall shear decreases further since the total area of tube contact becomes larger. However, the edges of the groove are then brought closer to the tube outside wall by such a concave shape which places limits on the maximum tube bite that may be taken by such a roll groove. If part of the tangential groove is made convex, the groove edges are removed further from the tube wall thereby allowing an increased tube bite without the edges of the groove scoring the tube surface.

Referring to FIG. 6, the various forces operating on a roll having groove characteristics of roll 79 (FIG. 5) when operating in a rolling mill described with respect to FIGS. 1-4, are shown. The roll has a center of rotation 107 and contacts a tube 100 with a rolling radius r that varies between R and R A roll trunnion 101 is contacted by its associated cam at a point 103 and the cam imparts a downward force thereon plus a horizontal frictional force, as shown by the arrows in FIG. 6 at the point 103. The roll trunnion 101 is subjected to restraining forces indicated by arrow 104 by the roll housing in which the roll is journaled. The trunnion forces 104 may be in the direction shown or may be in an opposite direction. The roll of FIG. 6 is illustrated as biting into the wall of the tube 100 an amount 8 by contact along the line 105 of the roll groove. The tube reacts against the roll with a force indicated at 105 by its horizontal component h and its vertical component v. The tube 100 is held by a mandrel within the tube. The reactive horizontal force component h is a result of the tube 100 being thrust by the roll bite B in the same direction in which the roll is traveling. If the roll reciprocates back and forth over the tube, the reversal of the horizontal force h tends to cause oscillatory motion in the tube and the mandrel along their length. Such oscillatory motion is undesirable because of added tool stresses created thereby. Such tool stresses limit the amount of bite that can be taken by the rolls and thus limit the amount of tube reducing that can be accomplished in a given amount of time. Therefore, it is desirable to minimize the oscillatory thrust of the rolls against a tube being reduced in order to increase production of a tube reducing mill.

A technique for minimizing such forces on the tube and various machine parts is to adjust the trunnion radius R, to provide a reactive torque force 7 by the tube in a direction opposite to that of the reactive force h caused by the bite B. The torque force 1- (FIG. 6) is a frictional force between the tube and the roll. It will be noted that although the tube contacting roll radius r of FIG. 6 varies between R and R during the tube reducing stroke, the trunnion radius remains a constant. Therefore, there will be some torque (tangential) force applied by the roll to the tube for at least a portion of each rolling stroke since the torque force can be zero only when there is a proper match between r and the trunnion radius R That is, the roll will tend to slip against the tube 100 at any point thereof an amount dependent on the rolling radius r at that tube point.

For the particular roll housing and cam housing driving arrangement shown in FIG. 1,

roll housing stroke/cam working length diameter of pinion gear ZS/diameter of pinion gear 26 N 1 The quantity N is defined in equation 1 as a convenience so the relationship between a trunnion radius R and the rolling radius r as it affects a torque force applied to the tube may be determined. A quantity NR 1 is defined for convience as the effective roll trunnion radius. If the effective trunnion radius is less than the rolling radius r at some point along the tube during the reducing stroke, the tube is driven by a torque force in a direction opposite to that being traveled by the roll. Conversely, if the effective trunnion radius is greater than r at some point along the tube, it is driven by a torque force in the same direction in which the roll is traveling. Finally, when the effective trunnion radius is equal to r there is no thrust applied to the tube due to torque forces. A significant part of the present invention is to balance tube thrust caused by the roll bite against tube thrust imparted by rolling torque.

For each of the second stage rolls the rolling mill of FIGS. 1-4, including roll 45, the effective trunion radius, NR is preferably made substantially equal to the minimum rolling radius, R The torque force works to balance the thrust given the tube by the roll bite. Referring to FIG. 4, the roll 45 first contacts the tube at about location F. At this location, the rolling radius r is substantially R and, therefore, the mag nitude of the torque force is a maximum and in a direction opposite to the direction in which the roll is traveling. As the roll proceeds to location J, the rolling radius r becomes substantially R and the magnitude of the torque force will be about zero. For most applications, the effective trunnion radius NR is preferably made a few percent (()3%) less than R in order to provide a slight torque force at position J in a direction opposite to the direction in which the roll is traveling. Since the tube is elongating, the effective trunnion radius NR J45 may even be slightly greater than R in certain cases so long as roll groove surface displacement at position I relative to the tube surface is in a direction to the left in FIG. 4 while the roll is proceeding to the right. These considerations apply to a single stage rolling mill or to the second stage of a two stage rolling mill.

It has already been pointed out that the torque forces applied to the tube by a roll vary over the rolling stroke as the rolling radius r varies. It may be noted that the forces applied to the tube by a roll bite also vary somewhat in magnitude over a rolling stroke as the roll bite B varies. Therefore, the two forces do not necessarily cancel each other completely but the criteria for trunnion radius design described herein can minimize the resultant thrust on a tube by application of such forces. By minimizing the resultant longitudinal force on the tube, the tube is more easily held in place against oscillatory movement along its length. This has an advantage that the tube is reduced in a more controlled manner, resulting in a better quality reduced tube.

The discussion hereinbefore with respect to FIG. 6 has not considered the fact that the tube material is flowing or elongating with respect to the mandrel under the influence of the rolls working to produce either a wall thickness reduction or a significant means diameter reduction, or both. The degree of metal flow or elongation can be seen to cumulatively increase along the mandrel or stroke length during the progressive wall and/or diameter reductions. If two sets of rolls, as described herein with respect to FIGS. l-4, are utilized, then while the centers of each of the rolls are driven at the same velocity relative to the mandrel by a journaled connection with a common roll housing, the tube portions independently contacted by each of the two sets of rolls must be moving at different velocities relative to the tube mandrel. The portion of the tube contacted by the second stage rolls (45, 47, 49) moves faster (more elongation) than the portion of the tube contacted by the first stage rolls (39, 41, 43). Therefore, the effective roll trunnion radius of each roll of one set is also chosen relative to the effective trunnion radius of each roll of the other set in order to take into account the different flow velocity of the tube surface areas contacted simultaneously by rolls of each of the roll sets.

A specific technique will now be described for designing the roll trunnion radii for each of the rolls of the rolling mill described with respect to FIGS. 1-4. The second stage rolls 45, 47 and 49 each have an effective roll trunnion radius as described hereinabove. A procedure for determining an optimum trunnion radius for each of the first stage rolls 39, 31, and 43 will be described which results in minimizing longitudinal forces on the tube being reduced.

Referring to FIG. 4, roll 39 is at position Bl when the roll 45 first contacts the tube at position F. Similarly, when the roll 39 is at position F and leaving contact with the tube, the roll 45 is at the position G1. The section B1-Gl of the tube is of concern since this section may be placed in tension or compression between the rolls 39 and 45. The rolls 39 and 45 are separated at their centers by a constant distance L since they are both held by the roll housing 23. Therefore,

L =BlF=F-Gl 2 At positions B, B1, F and G1, the tube being reduced has cross-sectional areas A A A and A respectively. These areas are determined by the specific shape of the mandrel and the cams associated with the rolls.

At the end of each rolling stroke, the tube 33 is fed to the right in FIG. 4 over the mandrel 37 in preparation for a new rolling stroke. The amount of feed may be denoted as f,, which indicates the amount of tube movement due to feed at position B. The material of the tube 33 at other positions along the tube 33 is caused to move to the right during each stroke of the roll an amount of the feed f plus additional elongation due to the fact that the tube material is caused to flow to the right as the tube wall thickness and inside diameter are reduced. The total elongation at various positions along the tube may be denoted as f,;,, f and f for instance. Each of these elongation quantities is related to a corresponding cross-sectional area of the tube in terms of tube volume as follows:

fB B fBi B1 fF F fGl G1 This relationship follows since tube material is not lost during the rolling process. The volume of metal does not change during the process but is merely redistributed into a different shape.

As roll 39 moves from position B1 to position F, the tube surface over which it travels moves in a direction of rolling an amount equal to fp f 1. Also, as roll 45 moves from position F to position G1, the tube surface over which it travels moves in the direction of rolling an amount f f plus an amount f -f Therefore, the total elongation E between the sets of rolls is as follows:

fB B( m c1) i;i ci Equation 5 results from combining equations 3 and 4.

The distance between the sets of rolls, L may be expressed in terms of each rolls effective trunnion radius and the number of revolutions taken to travel a given linear distance along the cams. This may be expressed as follows:

45 a ts where r and r represent average rolling radii of the rolls 39 and 45 against the tube, respectively. The average rolling radius is calculated as an arithmetical average between R and R of each roll.

It is helpful to define a quantity E as the elongation capability of the two sets of rolls. That is, E is that elongation that may take place in the tube during the rolling process which will not result in any slippage of the rolls against the tube relative to one another. To express it another way, E is that elongation of the tube which will result in the rolls placing a tube section between the rolls neither in compression nor tension. The elongation capability that the rolls must have can be shown to be the following:

ELR=L39 L45 If equations 6 and 8 are combined in a manner to eliminate their common term and if equations 7 and 9 are combined in a manner to eliminate their common term (0 and, further, if each of these resulting equations is solved for L and L the resulting equations may be substituted into equation 10 which will result in the following expression:

a N m m Therefore, equation 11 defines in terms of fixed parameters of the tube rolling machine a capability of the rolls of that machine to operate without slipping against the tube and thus without the undesirable forces of compression or tension to which the tube may be subjected. In order that the slippage and thus these forces are made zero, the actual elongation of the tube E as defined in terms of other parameters of the machine according to equation 5, must be equal to the elongation capability of the rolling mill as defined by equation 11. The following equation combining equations 5 and l 1 sets forth this condition:

Equation 12 is solved for the single unknown R It will be noted that the quantity R has been determined in a manner outlined hereinbefore for the second stage rolls in order to minimize thrust upon the tube. A desired feed f,; is assumed for the purpose of solving equation 12 for R,;,,,. The remainder of the quantities of equation 12 are physical factors of the rolling mill. Therefore, the trunnion radius R of each roll of the first set of rolls may be determined for a given feed f to bring about the desirable condition that the elongation capability of the two sets of rolls E is equal to the actual elongation of the tube E Due to a variety of conditions, the feed rate f,, is not conveniently maintained at the assumed value, especially during start-up operations of the machine. Obviously, it is inconvenient to change the trunnion radius of the first stage rolls 39, 41 and 43 each time the feed rate f is changed somewhat. It has been found as part of the present invention that the rolling mill may be operated with the actual elongation of the tube E being greater than the elongation capability of the rolls E Under such conditions, the two sets of rolls place the tube section therebetween under longitudinal compression. This causes the tube to become loose on the mandrel. The rolls are reactively thrust against their cams and supports until resistance to rotation develops at which point the rolls skid in relation to their cams with resultant harm to the various working surfaces. However, these undesirable results for a condition of E E are tolerable when the following conditions are met:

u: 5 ELT 2ELR (l To state the permissible variation another way, the feed rate can be increased by up to a factor of 2 from the assumedf for which the trunnion radius R J45 was calculated according to equation 12.

It has been found that the converse situation of B less than E is highly undesirable. When this occurs, the tube section between the rolls is in longitudinal tension and the tube contracts with resultant tightening on the mandrel. The resistance to rotation of the rolls which develops under these conditions causes a high reactive force 104 (FIG. 6) which results in early failure of the roll trunnion journals. The reason for this appears to be that the resistance to roll rotation causes the trunnion thrust (force 104) to be in the same direction as the driving torque applied by the cam to the roll through its trunnion (forces at 103). As the rolling stroke progresses, a mismatch between roll/tube elongation occurs, the trunnion force 104 increases, the horizontal cam driving torque force at 103 decreases and finally reverses in direction to cause slippage between the roll and cam or trunnion failure will occur. In the more permissible case of B being greater than E as noted with reference to equation 13, the resistance on a roll causes trunnion thrust to be in an opposite direction as the driving torque applied by the cam to the roll through its trunnion so that as trunnion thrust (force 104) increases, the roll merely slips against the cam without major harm.

It shall be understood that the invention is not limited to the specific arrangements shown, and that changes and modifications may be made within the scope of the appended claims.

What is claimed is:

1. In a tube rolling mill of the type wherein a plurality of grooved rolls are reciprocated along a working length of a mandrel in a manner that the grooves of each roll may contact and roll along a tube carried by the mandrel, said rolls being constrained to follow a predetermined path relative to the mandrel, the improvement wherein the groove of each of said rolls is of uniform radial cross-sectional shape around an entire circumferential surface of the roll, said groove shaped in radial cross-section having an arcuate line in a center portion thereof with a radius of curvature equal to or a few percent less than the smallest tube outside surface radius for which said predetermined roll path provides, said groove additionally including in cross-section symmetrical lines on either side of said arcuate line that are shaped to provide two zones of contact around the tube circumference along substantially the entire working mandrel length for which the mandrel and roll paths are adapted to receive and reduce the tube.

2. A tube rolling mill according to claim 1 wherein said mandrel is tapered along its said working length in order to provide significant tube inside diameter reduction as well as wall thickness reduction.

3. A tube rolling mill according to claim 1 wherein said symmetrical lines on either side of said arcuate line in groove radial cross-section are substantially straight lines tangent to said arcuate line.

4. In a tube rolling mill characterized by a tube supporting mandrel, a roll housing surrounding said mandrel and adapted to reciprocate over a defined stroke distance along the length of said mandrel, a plurality of rolls journaled in said roll housing in a manner to be free to rotate about their axes which lie in a common plane of the housing that is substantially perpendicular to said mandrel, said rolls arranged to contact said tube simultaneously, each of said rolls having its largest radius circumferential surface with a tube contacting groove of uniform shape around said roll, a pair of cam surfaces associated with each of said rolls and adapted to be reciprocated along the length of said mandrel a cam working length distance that is a constant multiple of the roll housing stroke distance, each of said rolls including a pair of trunnions on opposite sides of its said largest radius circumferential surface for hearing against its associated pair of cams in a manner to be guided thereby so that each roll groove follows a predetermined path relative to the mandrel, thereby to control the portion of the roll housing stroke distance in which the rolls will contact the tube and also to control the amount of roll bite into the tube and thus tube wall thickness reduction which results from reciprocating said rolls over the tube, the improvement wherein each of said roll grooves is shaped to have two zones of contact over substantially the entire portion of the roll housing stroke distance in which the rolls will contact the tube, thereby having a variable effective roll groove radius between a minimum and a maximum along the tube length, thus causing torque forces over at least a portion of the tube contacting length of the rolls that impart longitudinal forces to the tube, the further improvement wherein the radii of each pair of roll trunnion surfaces are of a value to balance against each other the roll bite and torque forces imparted to said tube, whereby the maximum longitudinal forces of the tube caused by the rolling are minimized.

5. A tube rolling mill according to claim 4 wherein the radii of each pair of said roll trunnions is substantially equal to the minimum effective roll groove radius divided by said constant multiple relating the cam working length and roll housing stroke.

6. A tube roiling mill according to claim 4 wherein said mandrel is significantly tapered along a portion of its length where a tube carried thereby is contacted by said rolls, thereby to provide significant tube inside diameter reduction as well as wall thickness reduction.

7. In a tube rolling mill characterized by a tube supporting mandrel a roll housing surrounding said mandrel and adapted to reciprocate over a defined stroke distance along the length of said mandrel, a plurality of rolls in two clusters journaled in said roll housing in a manner to be free to rotate about their axes, the axes of a first cluster of rolls lying in a first plane of the housing, the axes of a second cluster of rolls lying in a second plane of the housing, said first and second planes being substantially perpendicular to said mandrel and separated a fixed distance in said roll housing, each of said rolls having its largest radius circumferential surface provided with a tube contacting groove of uniform shape around said roll, a pair of cam surfaces associated with each of said rolls and adapted to be reciprocated along the length of said mandrel a cam working length that is a constant multiple of the roll housing stroke distance, each of said rolls including a pair of trunnions on opposite sides of its said largest radius circumferential surface for bearing against its associated pair of cams in a manner to be guided thereby so that each roll groove follows a predetermined path relative to the mandrel, thereby to control the portion of the roll housing stroke distance over which the rolls will contact the tube and also to control the amount of roll bite into the tube and thus tube reduction which results from reciprocating said rolls over the tube, said rolling mill additionally characterized by means for advancing tubing along the mandrel an increment of feed each roll housing'stroke, the improvements comprising,

a taper on said mandrel such that its largest diameter end in oriented for only the first cluster of rolls to contact at the larger end a tube carried by the mandrel and its smaller diameter end is oriented only for the second cluster of rolls to contact at the smaller end a tube carried by the mandrel, said smaller diameter being substantially equal to the desired inside diameter of a finished tube,

each of said roll grooves shaped to have two zones of contact over substantially the entire portion of the roll housing stroke distance in which the roll will contact the tube, thereby having a variable effective roll groove between a minimum and a maximum during its contact along the length of the tube, thus causing torque forces over at least a portion of a length of the tube for which each roll is adapted to contact which impart longitudinal forces to the tube, and

the radii of each pair of trunnion surfaces of each roll in said second cluster being of a value to drive the tube in a direction opposite to the direction of rolling for substantially the entire length of the tube that the roll is adapted to contact, whereby the maximum longitudinal forces transferred from the second cluster of rolls to said tube are minimized.

8. A tube rolling mill according to claim 7 with the further improvement wherein the radii of each pair of trunnion surfaces of each roll in said first cluster is of a magnitude so that the two clusters of rolls when both are in contact with a tube would tend to elongate the tube between them if there were no slippage of the rolls on the tube, said elongating capability of the rolls equal to or slightly less than the actual elongation of the tube due to its reduction.

9. A tube rolling mill according to claim 8 wherein the radii of each pair of trunnion surfaces of each roll in said first cluster is such that the actual tube elongation for said increment of feed is equal to or greater than the elongation capability of the two clusters of rolls but less than twice said elongation capability.

10. A tube rolling mill according to claim 9 wherein the radii of each pair of trunnion surfaces of each roll in said second cluster is substantiaily equal to the minimum effective tube contacting groove radius of the roll divided by said constant multiple between the cam working length and roll housing stroke distance.

1 l. A tube rolling mill characterized by a plurality of rolls pivotally mounted in a roll housing which is reciprocated along a length of a tube, and further characterized by cam surfaces for rotatably guiding each roll against said tube with predetermined displacements, said cam surfaces being reciprocated in a direction along the length of said tube with a cam working length that bears a constant relationship to the roll housing stroke, the improvement comprising:

a tube contacting groove in an outer circumferential surface of each of said rolls, said groove having a uniform cross-section therearound, and

a trunnion surface on each roll for contacting said cam surface, the radius of said trunnion surface adjusted to minimize longitudinal forces along said tube.

12. A tube rolling mill according to claim 11 wherein the groove of each of said rolls is shaped in cross-section in a manner to contact said tube at two zones therearound throughout most of said length of tube.

13. A tube rolling mill according to claim 1 1 wherein said uniform groove includes in cross-section an arcuate line in the center thereof bounded on either side by substantially straight lines tangent to said arcuate surface, said arcuate line having a radius of curvature substantially equal to the smallest tube outer surface radius which said roll is designed to contact.

14. In the art of reducing metal tubing by use of an internal mandrel and external rolls, a method of reducing the internal diameter and wall thickness of a tube, comprising the steps of:

contacting the outside surface of the tube at two areas thereof with each of said rolls for substantially the entire time each of said rolls is biting into said tube, and applying torque to each of said rolls in an amount to minimize maximum longitudinal forces imparted to said tube as a result of said torque and said each of said rolls biting into said tube so as to balance the tube thrust caused by the roll bite against the tube thrust imparted by rolling torque.

15. In the art of reducing metal tubing by use of an internal mandrel and at least two sets of external rolls separated a fixed amount along the length of the mandrel, comprising the steps of:

reciprocating the sets of rolls relative to the mandrel,

moving one of said sets of rolls against the tube on the mandrel in a manner to effect primarily inside diameter reduction of the tube, and concurrently moving the other of said sets of rolls against said tube in a manner to effect primarily wall thickness reduction.

, gig) mm PA'iEN l OFHCE tmrmmm'rr or commcrmm Patent No. ,540 Dated p l9 72 Inventor-(s) Richard E. Russell It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

a j i Title page, after Inventor: "Richard E. Russel" should read "Richard E. Russell".

Column- 1, line 44, change "resulting" to "relating". Column ll, line 28, change "means!' to "mean" Column 16, line 14, change "largest" to "larger" line 15, change "in. to "is".

Signed and sealed this 22nd day of May 1973.

(SEAL) Attest:

- EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK" Attesting Officer Commissioner of Patents 

1. In a tube rolling mill of the type wherein a plurality of grooved rolls are reciprocated along a working length of a mandrel in a manner that the grooves of each roll may contact and roll along a tube carried by the mandrel, said rolls being constrained to follow a predetermined path relative to the mandrel, the improvement wherein the groove of each of said rolls is of uniform radial cross-sectional shape around an entire circumferential surface of the roll, said groove shaped in radial cross-section having an arcuate line in a center portion thereof with a radius of curvature equal to or a few percent less than the smallest tube outside surface radius for which said predetermined roll path provides, said groove additionally including in cross-section symmetrical lines on either side of said arcuate line that are shaped to provide two zones of contact around the tube circumference along substantially the entire working mandrel length for which the mandrel and roll paths are adapted to receive and reduce the tube.
 2. A tube rolling mill according to claim 1 wherein said mandrel is tapered along its said working length in order to provide significant tube inside diameter reduction as well as wall thickness reduction.
 3. A tube rolling mill according to claim 1 wherein said symmetrical lines on either side of said arcuate line in groove radial cross-section are substantially straight lines tangent to said arcuate line.
 4. In a tube rolling mill characterized by a tube supporting mandrel, a roll housing surrounding said mandrel and adapted to reciprocate over a defined stroke distance along the length of said mandrel, a plurality of rolls journaled in said roll housing in a manner to be free to rotate about their axes which lie in a common plane of the housing that is substantially perpendicular to said mandrel, said rolls arranged to contact said tube simultaneously, each of said rolls having its largest radius circumferential surface with a tube contacting groove of uniform shape around said roll, a pair of cam surfaces associated with each of said rolls and adapted to be reciprocated along the length of said mandrel a cam working length distance that is a constant multiple of the roll housing stroke distance, each of said rolls including a pair of trunnions on opposite sides of its said largest radius circumferential surface for bearing against its associated pair of cams in a manner to be guided thereby so that each roll groove follows a predetermined path relative to the mandrel, thereby to control the portion of the roll housing stroke distance in which the rolls will contact the tube and also to control the amount of roll bite into the tube and thus tube wall thickness reduction which results from reciprocating said rolls over the tube, the improvement wherein each of said roll grooves is shaped to have two zones of contact over substantially the entire portion of the roll housing stroke distance in which the rolls will contact the tube, thereby having a variable effective roll groove radius between a minimum and a maximum along the tube length, thus causing torque forces over at least a portion of tHe tube contacting length of the rolls that impart longitudinal forces to the tube, the further improvement wherein the radii of each pair of roll trunnion surfaces are of a value to balance against each other the roll bite and torque forces imparted to said tube, whereby the maximum longitudinal forces of the tube caused by the rolling are minimized.
 5. A tube rolling mill according to claim 4 wherein the radii of each pair of said roll trunnions is substantially equal to the minimum effective roll groove radius divided by said constant multiple relating the cam working length and roll housing stroke.
 6. A tube rolling mill according to claim 4 wherein said mandrel is significantly tapered along a portion of its length where a tube carried thereby is contacted by said rolls, thereby to provide significant tube inside diameter reduction as well as wall thickness reduction.
 7. In a tube rolling mill characterized by a tube supporting mandrel a roll housing surrounding said mandrel and adapted to reciprocate over a defined stroke distance along the length of said mandrel, a plurality of rolls in two clusters journaled in said roll housing in a manner to be free to rotate about their axes, the axes of a first cluster of rolls lying in a first plane of the housing, the axes of a second cluster of rolls lying in a second plane of the housing, said first and second planes being substantially perpendicular to said mandrel and separated a fixed distance in said roll housing, each of said rolls having its largest radius circumferential surface provided with a tube contacting groove of uniform shape around said roll, a pair of cam surfaces associated with each of said rolls and adapted to be reciprocated along the length of said mandrel a cam working length that is a constant multiple of the roll housing stroke distance, each of said rolls including a pair of trunnions on opposite sides of its said largest radius circumferential surface for bearing against its associated pair of cams in a manner to be guided thereby so that each roll groove follows a predetermined path relative to the mandrel, thereby to control the portion of the roll housing stroke distance over which the rolls will contact the tube and also to control the amount of roll bite into the tube and thus tube reduction which results from reciprocating said rolls over the tube, said rolling mill additionally characterized by means for advancing tubing along the mandrel an increment of feed each roll housing stroke, the improvements comprising, a taper on said mandrel such that its largest diameter end in oriented for only the first cluster of rolls to contact at the larger end a tube carried by the mandrel and its smaller diameter end is oriented only for the second cluster of rolls to contact at the smaller end a tube carried by the mandrel, said smaller diameter being substantially equal to the desired inside diameter of a finished tube, each of said roll grooves shaped to have two zones of contact over substantially the entire portion of the roll housing stroke distance in which the roll will contact the tube, thereby having a variable effective roll groove between a minimum and a maximum during its contact along the length of the tube, thus causing torque forces over at least a portion of a length of the tube for which each roll is adapted to contact which impart longitudinal forces to the tube, and the radii of each pair of trunnion surfaces of each roll in said second cluster being of a value to drive the tube in a direction opposite to the direction of rolling for substantially the entire length of the tube that the roll is adapted to contact, whereby the maximum longitudinal forces transferred from the second cluster of rolls to said tube are minimized.
 8. A tube rolling mill according to claim 7 with the further improvement wherein the radii of each pair of trunnion surfaces of each roll in said first cluster is of a magnitude so that the two clusters of rolls when both are in contact wiTh a tube would tend to elongate the tube between them if there were no slippage of the rolls on the tube, said elongating capability of the rolls equal to or slightly less than the actual elongation of the tube due to its reduction.
 9. A tube rolling mill according to claim 8 wherein the radii of each pair of trunnion surfaces of each roll in said first cluster is such that the actual tube elongation for said increment of feed is equal to or greater than the elongation capability of the two clusters of rolls but less than twice said elongation capability.
 10. A tube rolling mill according to claim 9 wherein the radii of each pair of trunnion surfaces of each roll in said second cluster is substantially equal to the minimum effective tube contacting groove radius of the roll divided by said constant multiple between the cam working length and roll housing stroke distance.
 11. A tube rolling mill characterized by a plurality of rolls pivotally mounted in a roll housing which is reciprocated along a length of a tube, and further characterized by cam surfaces for rotatably guiding each roll against said tube with predetermined displacements, said cam surfaces being reciprocated in a direction along the length of said tube with a cam working length that bears a constant relationship to the roll housing stroke, the improvement comprising: a tube contacting groove in an outer circumferential surface of each of said rolls, said groove having a uniform cross-section therearound, and a trunnion surface on each roll for contacting said cam surface, the radius of said trunnion surface adjusted to minimize longitudinal forces along said tube.
 12. A tube rolling mill according to claim 11 wherein the groove of each of said rolls is shaped in cross-section in a manner to contact said tube at two zones therearound throughout most of said length of tube.
 13. A tube rolling mill according to claim 11 wherein said uniform groove includes in cross-section an arcuate line in the center thereof bounded on either side by substantially straight lines tangent to said arcuate surface, said arcuate line having a radius of curvature substantially equal to the smallest tube outer surface radius which said roll is designed to contact.
 14. In the art of reducing metal tubing by use of an internal mandrel and external rolls, a method of reducing the internal diameter and wall thickness of a tube, comprising the steps of: contacting the outside surface of the tube at two areas thereof with each of said rolls for substantially the entire time each of said rolls is biting into said tube, and applying torque to each of said rolls in an amount to minimize maximum longitudinal forces imparted to said tube as a result of said torque and said each of said rolls biting into said tube so as to balance the tube thrust caused by the roll bite against the tube thrust imparted by rolling torque.
 15. In the art of reducing metal tubing by use of an internal mandrel and at least two sets of external rolls separated a fixed amount along the length of the mandrel, comprising the steps of: reciprocating the sets of rolls relative to the mandrel, moving one of said sets of rolls against the tube on the mandrel in a manner to effect primarily inside diameter reduction of the tube, and concurrently moving the other of said sets of rolls against said tube in a manner to effect primarily wall thickness reduction. 